(573ad) Systematic analysis on the energy transition pathways for sustainable natural gas production

According to the International Energy Agency, in the present context of market uncertainty, the natural gas prices are foreseen to remain extremely volatile [1], which reinforces the need of alternative pathways to overcome its limited availability and guarantee the supply of this important commodity. The combination of anaerobic digestion and thermal gasification ensures efficient use of biomass, since the unconverted biomass from digesters can be used to produce syngas through thermal gasification. On the other hand, hydrogen production from water electrolysis has recently drawn attention as a versatile solution for balancing intermittent renewable electricity generation, particularly from sources like wind and solar. In addition, reversible solid oxide cells (rSOCs) are an electrochemical energy conversion technology that can produce both electricity from fuel (gas-to-power) and fuel from electricity (power-to-gas), depending on resource availability and demand. Hydrogen is produced in an rSOC operated as an electrolyzer, using excess power from renewable resources. Next, it is combined with syngas from biowaste and residue gasification to produce methane. The rSOC system can also be operated in fuel cell mode by oxidizing methane to produce electricity and balance power requirements of the integrated system if renewable electricity is not available from the grid.

This work investigates the integration of biodigestion, gasification and rSOCs aiming to increase the sustainable natural gas yield, while evaluating the tradeoffs of minimizing both the CO₂ emissions and the total cost of the different configurations. In order to assess the impact of different scenarios for the energy transition in the sustainable natural gas production, the seasonal excess and deficit of electricity is considered, either in the current energy mix scenario and also for a future energy mix scenario, in which a full renewable grid is modeled based on the generation (taking into account GIS-based land-restriction, geo-spatial wind speed and irradiation data, and the maximum electricity production from renewable sources was derived considering EU-wide low restrictions) and demand (full electrification of the residential and mobility sectors is assumed) [2].

Given the fluctuating costs of seasonal energy and the intermittent availability of renewable energy sources, the integration of other technologies, such as CO₂ injection and storage systems (e.g., SNG, CO₂), are also assessed. The OSMOSE Lua platform is used to address the optimization problem, seeking to minimize energy consumption, the overall cost of the chemical plant, and also the CO₂ emissions. A mixed integer linear programming (MILP) method is employed to identify optimal system configurations under different economic scenarios. An incremental financial analysis that incorporates the uncertainty related to the acquisition and selling costs of the feedstock and fuels produced, when embedded in a volatile market, by using the Monte Carlo method is performed. This methodology helps elucidating the feasibility of the proposed processes.

The proposed superstructure is shown in Fig 1. The biodigestion process is modelled considering a biomethane potential of 300 Nm³ CH₄ per t of volatile solids using organic wastes [3]. A water scrubbing technology is considered for the biogas upgrading, as it can obtain methane concentration of > 99% CH₄ consuming around 0.20 kWh/Nm³. The CO₂ is also recovered in the upgrading system with a purity above 99.5% [4]. Upgraded biomethane is marketed and the CO₂ rich stream follows to the biomethane production. The CO₂ from the anaerobic digestion unit could be liquefied and stored in a tank at -50 °C and 7 bar (1,155 kg/m³). Liquefied CO₂ can be later regasified and fed to a methanation system, in which the hydrogen necessary is provided by the rSOC system under the electrolyzer mode, operating at 1 bar, 800 °C with steam conversion rate of 81%. The rSOC system can also be operated in fuel cell mode by oxidizing methane to produce electricity. The rSOC system is modelled considering the concentration, ohmic and activation overpotentials [5].

The gasification of the digestate coming from the anaerobic digestion unit occurs at atmospheric pressure and uses steam as gasification agent. After leaving the gasifier, the syngas is treated to remove tars and impurities. A fraction of the char produced in the pyrolysis step is combusted to supply the heat required by the endothermic drying, pyrolysis and reduction reactions. Hydrogen coming from SOEC is used to adjust syngas composition for the methanation reaction ((n_H2-n_CO2)/ (n_CO+n_CO2) = 3.1).

The methanation system is based on the TREMP® process [6], in which a series of methanation beds are intercooled by recycling or indirect inter-cooling in order to achieve higher conversion. Simulations are performed in Aspen Plus® software, using Peng-Robinson EoS with Boston-Mathias modifications.

A systematic approach combines different energy solutions and time-varying energy demands to identify operating conditions and arrangements for maximizing the sustainable natural gas production. The storage systems enabled the seasonal storage of the CO₂ streams, which were utilized to produce CH₄ when renewable electricity is available (see Fig 2). CH₄ is stored and used to generate power to balance the system demands via SOFC. This strategy enables harvesting excess renewable electricity and producing value added fuels from biogenic emissions. Availability of renewable electricity impacts not only overall generation and demand balance, but also extent of use of the CO₂ and the performance of its management system to produce value-added products and handle the intermittency of renewable sources. The uncertainty of the market prices for the commodities has been also considered, revealing that the use of constant inputs costs may lead to misleading results regarding the likelihood of loss of risky projects. The implementation of more rigorous carbon taxes and the technologies maturation may reduce the financial risks and give support to decision-makers in implementing biorefineries and exploring new business opportunities to trigger the decarbonization of important commodities.

References

1. IEA. Russian supplies to global energy markets. International Energy Agency; 2022 Available from: https://www.iea.org/reports/russian-supplies-to-global-energy-markets Accessed 17 March 2024
2. Santecchia A, Castro-Amoedo R, Nguyen TV, Kantor I, Stadler P, Maréchal F. The critical role of electricity storage for a clean and renewable European economy. Energy & Environmental Science. 2023;16(11):5350–70.
3. Wellinger A, Murphy JD, Baxter D. The biogas handbook: science, production and applications. Elsevier; 2013.
4. Lems R, Langerak J, Dirkse E. “Next generation biogas upgrading using highly selective gas separation membranes” Showcasing the Poundbury project. DMT Environmental Technology: Joure, The Netherlands. 2008;
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6. Topsoe, H. From solid fuels to substitute natural gas (SNG) using TREMP Topsøe Recycle Energy-efficient Methanation Process. 2009; Available from: https://www.netl.doe.gov/sites/default/files/netl-file/tremp-2009.pdf Accessed 17 March 2024